Monte-Carlo simulation studies of the nonlinear imaging of a two dimensional surface wave field by a synthetic aperture radar

Abstract

The imaging of ocean surface waves by synthetic aperture radar (SAR) is investigated using two-dimensional Monte-Carlo simulations. The properties of the SAR imaging mechanism for windseas and swell in the Bragg scattering regime are discussed as a function of a few governing non-dimensional parameters formed from a combination of SAR and ocean wave parameters. The parameter ranges may be classified into three regimes corresponding to linear and weakly nonlinear, medium nonlinear and strongly nonlinear imaging. The nonlinearities are induced by motion effects (velocity bunching, velocity spread and acceleration smearing), while the real aperture radar (RAR) tilt and hydrodynamic modulation processes are regarded as linear. In the strongly nonlinear imaging regime, the velocity bunching mechanism causes a rotation of the spectral peak towards the range direction and a stretching of the peak wavelength. In addition, the azimuthal resolution is degraded through the Doppler spreading arising from the different facet velocities within a SAR resolution cell. The imaging properties in this regime are largely governed by two non-dimensional parameters, the velocity bunching and velocity smearing parameter. The nonlinear imaging distortions are strongest for broad spectra (windseas) and are significantly weaker for narrow-band swell. In the linear and weakly nonlinear imaging regime, the superposition of the hydrodynamic and tilt cross-section modulation and the velocity bunching transfer function normally produces a rotation of the spectral peak towards the azimuthal direction. The interference characteristics of these different modulation mechanisms depends on the wave propagation direction and can lead to a significant distortion of the image. This is often seen in large differences in the image modulation depths of waves propagating parallel and anti-parallel to the flight direction.     

Cover photograph Image from GOES-E spacecraft

Azimuthally travelling ocean waves are seldom well imaged by microwave real aperture radar (RAR) operating with conventional HH or VV polarizations. Attenuation of image intensity modulation in the azimuthal direction implies that ocean wave spectra derived from such images also will not be accurate. Real aperture radar cross-section modulation by long ocean waves is normally attributed to two principal sources, tilt modulation and hydrodynamic modulation. In ocean radar images both of these modulation sources are significantly attenuated in the azimuthal direction. Therefore, complete two-dimensional k-space wave spectra derived from microwave data often are quite different than physical ocean spectra. This paper uses fully-polarimetric radar measurements of ocean backscatter to identify a new source of backscatter modulation that is strongest in the azimuthal direction. This modulation source has potential for augmenting tilt and hydrodynamic modulation sources in the azimuthal direction where their weakness causes poor wave visibility. The predicted improvement in the measurement of ocean wave spectra using an optimized polarization is investigated by means of a RAR ocean imaging model. Fully-polarimetric, and conventional, radar spectrometers are proposed which are specifically optimized to sense wave-tilts in the azimuthal direction.     

High-resolution satellite measurements of coastal wind field and sea state

Methods to derive wind speed and sea state by simple empirical models from synthetic aperture radar (SAR) data are presented and applied for use in high-resolution numerical modelling for coastal applications. The new radar satellite, TerraSAR-X (TS-X), images the surface of the sea with a high resolution up to 1 m. Therefore, not only wind information and integrated sea state parameters but also individual ocean waves with wavelengths down to 30 m are detectable. Two-dimensional information on the ocean surface retrieved using TS-X data is validated for different oceanographic applications: derivation of finely resolved wind fields (XMOD algorithm) and integrated sea state parameters (XWAVE algorithm). Both algorithms are capable of taking into account fine-scale effects in coastal areas. Wind and sea state information retrieved from SAR data are applied as the input for a wave numerical spectral model (wind forcing and boundary condition) running at a fine spatial horizontal resolution of 100 m. Results are compared to collocated buoy measurements. Studies are carried out for varying wind speeds and comparisons against wave height, simulated using original TS-X-derived wind data, showing the sensitivity of waves to local wind variation and thus the importance of local wind effects on wave behaviour in coastal areas. Examples for the German Bight (North Sea) are shown. The TS-X satellite scenes render well-developed ocean wave patterns of developed swell at the sea surface. Refraction of individual long swell waves at a water depth shallower than about 70 m, caused by the influence of underwater topography in coastal areas, is imaged on the radar scenes. A technique is developed for tracking wave rays depending on changes in swell wavelength and direction. We estimate the wave energy flux along wave tracks from deep water to the coastline based on SAR information: wave height and wavelength are derived from TS-X data.     

Development of CFRP racing motorcycle rims using a heuristic evolutionary algorithm approach

The scope of this paper is the application of evolutionary optimization methods to the development of composite fibre reinforced plastics (CFRP) racing motorcycle rims. The mass and the moment of inertia of a front and a rear CFRP rim are minimized subject to manufacturing, strength, and stiffness constraints. The stacking sequence of the composite laminates is optimized by applying a sophisticated parameterization concept making an excellent compromise between a huge variety of structure properties and a reasonable number of optimization parameters. The mechanical properties are simulated using the finite element analysis package ANSYS . Resulting displacement and Tsai–Wu index values are combined with the mass of the rim in order to assign a fitness value to each different design solution. The smart formulation of the fitness function allows the exploration of solutions close to the required strength and stiffness properties. The proprietary software DynOPS is utilized as an optimization engine. It links an evolutionary algorithm to arbitrary simulation programs and controls the entire optimization process. The sophisticated parameterization concept, together with the fitness function formulation, are the basis for the development of CFRP motorcycle rims decisively lighter than state-of-the-art magnesium alloy rims.     

Synthetic aperture radar imaging of ocean surface waves: multi-look and single-look systems

A single two-dimensional expression is given for the (speckle-averaged) image associated with a model of ocean swell which may be applied to single-look and multi-look synthetic aperture radar (SAR) systems (albeit with different arguments). That expression accommodates real and artificial cross-section modulation mechanisms, shows rescaling effects (i.e. rotation of the image of a wavefield). suggests the use of processor refocusing to maximize image contrast, accounts for scene coherence, and indicates a variation of image contrast with range (assuming homogeneous swell). Most of the above have been observed and discussed previously in the literature. However, in this paper it is shown that all the effects are derived (simultaneously). by a relatively simple, rigorous and self-consistent analysis: it is not always necessary to treat the SAR imaging of ocean waves in a piece-wise manner. The analysis differs from that most frequently found in the literature. A space-time variant point spread function is not presumed; and some of the conclusions, in particular with regard to the optimum image processing strategy, differ with those following the use of the more conventional forms of analysis. Throughout, care is taken to minimize, but also to make explicit, the underlying assumptions in the treatment.     

The Benjamin–Feir instability and propagation of swell across the Pacific

About 40 years ago, Snodgrass and other oceanographers (1966) tracked ocean swell propagating across the entire Pacific Ocean. At about the same time, several investigators (including Benjamin and Feir) showed that a uniform train of plane waves of finite amplitude on deep water is unstable. Comparing these two results, each of which is highly cited, leads to the following question: in light of this instability, how did the waves tracked by oceanographers travel coherently more than 10,000 km across the Pacific Ocean? A possible explanation is provided in recent work that re-examined the Benjamin–Feir instability in the presence of linear damping. The conclusion was that even small amounts of damping can stabilize the instability before nonlinear effects become important. In addition, the theoretical predictions agree well with results from laboratory experiments. In this paper we re-examine ocean data from 1966 to estimate whether the oceanic damping that was measured could have controlled the Benjamin–Feir instability for the swell that was tracked. We find that for one set of ocean swell, dissipation controls the Benjamin–Feir instability enough to allow coherent wave propagation across the Pacific. For a second set of ocean swell, it does not. For a third set of ocean swell, an integral that the theory predicts to be constant is not constant in the data; it decreases and correspondingly the spectral peak shifts to a lower frequency—this is frequency downshifting. For this case the theory is not an adequate model, so the corresponding Benjamin–Feir analysis can be misleading. Thus, our results from the historical records are inconclusive: we can assert neither that dissipation of ocean swell is always negligible, nor that it is always important. But our results show that dissipation can control the Benjamin–Feir instability for small-amplitude waves and that downshifting occurs in ocean swell with relatively small wave slopes.     

Decision support system development for integrated management of European coastal lagoons

In this paper, a general framework for the development of Decision Support Systems (DSSs) for the management of coastal lagoons is presented. The proposed DSS structure integrates the information provided by several models accounting for different characteristics of lagoon ecosystems, including biogeochemical, hydrodynamic, ecological and socio-economic aspects. Outputs and indicators provided by the models are used to accomplish the decision task by the application of multicriteria analysis. Model uncertainty and robustness with respect to uncontrollable factors are addressed. Application of the proposed DSS structure to five lagoons located in the Mediterranean area is discussed, with special focus on the management of clam farming in the Sacca di Goro lagoon (Italy). Thanks to its flexibility, the proposed DSS structure is also applicable in decision problems arising in different fields.     

Numerical modeling of the impact of the Ancão Inlet relocation (Ria Formosa,Portugal)

This work describes the application of hydrodynamic (ELCIRC) and transport (VELA and VELApart) models to the Ria Formosa lagoon (Portugal) to study the impact of the relocation of the Ancão Inlet. Located in the south of Portugal, this lagoon is a mesotidal barrier island system that communicates with the sea through 6 inlets. The Old Ancão Inlet was artificially closed and the New Ancão Inlet was relocated into a westward position. This work investigates the hydrodynamic patterns and the potential pathways of tracers in Ria Formosa in two distinct configurations: before and after the Ancão Inlet relocation. The hydrodynamic model was successfully calibrated and validated against elevation, velocity and inlet discharges data, accurately reproducing the tidal propagation. The inlet relocation increases the magnitude of tidal currents, residual velocities and the tidal prism across the bar, suggesting a better stability. The tracers transport simulations suggest enhanced water exchanges through the Ancão Inlet and smaller residence times in the western part of Ria Formosa with the present configuration. Overall, it is concluded that the Ancão Inlet relocation had a positive contribution towards increasing the water renewal of the western part of the lagoon, thus decreasing its vulnerability to pollution.     

基于目标远程探测的合成孔径处理算法研究

给出了基于目标远程探测技术的BDSAP合成孔径处理算法,并且在消声水池中进行了8阵元合成48阵元以及单阵元合成8阵元的逆合成孔径实验研究。结果表明合成孔径处理与常规物理孔径处理相比具有较好的鲁棒性,并且在空间相关长度大于合成孔径长度以及信号时域相关长度大于合成孔径所需时间的水下或海洋环境里,合成阵增益与等长的物理阵增益基本相等。     

基于机载X-SAR海浪监测系统的设计

基于机载X波段合成孔径雷达设计了一套低空机载海浪监测系统.首先在对海浪微波散射和机栽合成孔径雷达成像原理进行分析的基础上,设计出海浪监测系统的总体架构;其次研究合成孔径雷达的成像算法,通过建立仿真模型,反演出海浪谱,最后测量出海浪浪高、海浪波长等主要信息,提供给飞行员用来判断飞机是否具备在海面上降落的安全条件.同时,对...     

Synthetic aperture radar observations of interaction of OCE'an swell and Semidi Islands in the Gulf of Alaska

Synthetic aperture radar (SAR) images of the ocean surface can show evidence of the sheltering elTects of barriers, such as islands, on the incoming long ocean waves. In the presence of high background noise in SAR images, these elTects arc not always observed by visual inspection. As shown in this paper, they can be determined most effectively in the spectral domain. From the ERS–1 SAR data of 17 October 1991, we noticed that two separate swell components almost perpendicular to each other were incident on the Semidi Islands in the Gulf of Alaska. By spectral analysis, the shadow zone associated with each wave component can be identified. The distribution of an equivalent dilTraetion coefficient has been estimated at the Ice of the Chowiet Island which has a sharp tip. It has been compared to that obtained from the Sommerfeld solution of dilTraction for a semi-infinite barrier. The SAR data shows a similar dilTraction pattern to the model solution but has a slower fall-off into the geometric shadow zone.     

小波离散点插值处理在综合孔径微波辐射计中的应用

土壤湿度和海水盐度是影响全球气候和水体循环的重要因素,但是对土壤湿度和海水盐度的测量都需要较低的微波测量频率,这时天线孔径和空间分辨率的矛盾就更加突出,而应用综合孔径微波辐射计就是在减小天线孔径和重量的前提下提高空间分辨率的一项新兴的被动微波遥感技术。综合孔径微波辐射计测量的是视场范围内亮温分布对于天线阵中不同基线的干涉天线单元之间的可见度函数分量,根据测得的可见度函数可以反演得到亮温图像。在综合孔径微波辐射计设计中,天线阵列设计和成像反演算法是两项关键技术。提出一种基于小波变换的离散点插值方法,它对二维综合孔径微波辐射计的Fourier成像反演算法进行了修正处理。     

深海采矿扬矿系统水下构形与动力学特性数值仿真

深海多金属结核开采系统的扬矿系统主要由扬矿硬管、扬矿泵、中间仓、扬矿软管等组成的长管线系统构成,其水下运动学和动力学行为,决定了深海采矿的成败.运用非线性有限元方法,建立了基于1000m深海采矿试验扬矿系统的有限元模型,进行了系统水下空间构形的静态和动态数值仿真,计算分析了海洋环境中的波流载荷,软管长度,软管集中吊挂浮力配置,以及采矿船和集矿机的运动状态等因素,对系统空间构形,尤其对软管形态的影响.水下管线的液动力载荷采用Morison公式计算.     

Multi-look processing of synthetic aperture radar data from dynamic ocean surfaces

The basic theory is described of multi-look processing of synthetic aperture radar (SAR) data from dynamic ocean surfaces. The principal effects inherent to multi-look SAR on the impulse response function are first illustrated using a moving point target model. The theory is then extended to the multi-look imagery of diffusely scattering ocean surface waves. Finally, the use of a sliding synthetic aperture is suggested to investigate the short life-times of small scale surface waves that are considered to be the predominant scatterers on the ocean surface.     

A finite element ecological model: a first application to the Venice Lagoon

A Finite Element Ecological Model (FEEM) has been set up by fully coupling a primitive equation finite element hydrodynamic model (FEM) to an ecological model derived from EUTRO, the ecological submodel contained in the water quality model WASP, released by US-EPA. The work required the ‘decoupling’ of EUTRO from WASP and its transformation into a standalone program, the testing of such a structure versus the results obtained by running the WASP in the 0D version, the improvement of its parameterisation up to the inclusion of zooplankton as a dynamic variable explicitly modelled, the definition of an interface module, and the actual coupling of the improved ecological module and the hydrodynamic primitive equation model. The integrated model has then been applied, as a first test, to the Venice Lagoon (VELFEEM). To this aim idealised forcings (tide, meteo) have been used together with actual field data (river discharge, nutrient loadings) to run simulations over a one-year period. Analysis of the results shows that the model is numerically stable, variations in the state variables are consistent, and nutrients, plankton and oxygen evolves in space and time in a ecologically coherent way. Main features of the lagoon, namely oxygen seasonal evolution and dispersion of nutrients from the point sources are qualitatively reproduced.An additional result is that the improved version of the 0D ecological submodel has been organised as an independent module which can be run by a single subroutine call, and therefore can be easily integrated in any structure and coupled to any hydrodynamic model. The coupling here presented demonstrates the ease of such an integration.     

干涉合成孔径雷达海浪遥感研究

海浪是重要的海洋环境参数, 它对港口建设、海上航行、油气田开发等具有重要的意义。合成孔径雷达(Syn thet ic Apertu re Radar, SAR ) 是目前大范围测量海浪的主要工具, 用它来估计海浪主波系统的波长和传播方向比较好, 但要定量的提供海浪的振幅以及SAR 图像强度与实际海面的关系是相当困难的, 而且由SAR 图像获取海浪方向谱也比较复杂。沿航迹干涉合成孔径雷达(Along-Track Interferometric Synthet ic Aperture Radar,ATI-SAR ) 是在平台飞行方向上安置两根天线的双天线合成孔径雷达。相对于合成孔径雷达, 真实孔径雷达调制传递函数对沿航迹干涉合成孔径雷达相位图像的影响很小, 因此沿航迹干涉合成孔径雷达更适合测量海浪。详细介绍了沿航迹干涉合成孔径雷达海浪遥感的成像机制, 为进一步反演海浪方向谱奠定了基础, 最后针对目前研究存在的问题以及今后有待研究的方向进行了探讨。     

应用Radon变换方法检测窄V形船舶航迹

ＳＡＲ航天遥感图像中的舰船航迹影象包含有舰船的进行速度、行进行方向、船体长度等重要信息,而充分开发利用与运动舰船航迹相关的信息资源,能够加强对上舰船航行的监测能力。通过对舰船航迹在卫星遥感ＳＡＲ图象中不同模式及Ｒａｄｏｎ变换原理和特性的分析,提出了应用Ｒａｄｏｎ变换对ＳＡＲ图象中的窄Ｖ形舰船航迹进行了检测的方法,通过实例测试,效果比较理想。     

全极化合成孔径雷达海浪遥感方法

提出了一种利用全极化真实孔径雷达测量方位方向和距离方向海浪斜率的新方法,该方法不同于极化方向角估计海浪方位向斜率方法,利用两种线极化图像信息将有关弱极化项剔除(流体动力学调制),获得仅包含倾斜项和极化调制项的雷达成像公式。利用机载SAR取代真实孔径雷达进行海浪反演,获得了与浮标致的结果。另外,在速度聚束模式情况下,推导了双极化合成孔径雷达图像谱同海浪谱的非线性变换关系。     

极化合成孔径雷达测量海浪的研究

描述了极化合成孔径雷达海浪成像过程。提出了一种极化雷达反演海浪方向谱的方法,该方法利用合成孔径雷达图像谱与海浪方向谱之间简单的线性关系和最优方法反演得到海浪方向谱,并将结果与现场资料进行了比较。经过对同一海区机载雷达P、L、C 3个波段的图像进行反演,其方向谱相当一致,而且有效波高和波向与实测数据吻合相当好。